Working Fluid For An Orc Process, Orc Process and Orc Apparatus

ABSTRACT

The invention relates to working fluids for energy conversion in a thermal ORC process for combined generation of electrical and heat energy. The heat source used is in particular thermal water. The working fluids used are partially or perfluorinated hydrocarbons and/or partially or perfluorinated polyethers and/or partially or perfluorinated ketones. In one embodiment of the invention, the working fluid used is a combination of 1,1,1,3,3-pentafluorobutane and a fluorinated polyether having a molecular weight of 340 and a boiling point of 57° C. at 101.3 kPa.

The invention relates to working fluids for an ORC process for energyconversion in a thermal cycle process for combined generation ofelectrical and heat energy, to an ORC process and to an apparatus forperforming an ORC process.

The ORC (organic Rankine cycle) process is a thermodynamic cycle namedafter William Rankine. ORC installations convert thermal energy intoelectric current. In a thermal ORC process, selection of the appropriateworking fluids allows temperature differences to be bridged andutilized. Such plants are utilized for the electrical energy generationfrom the waste heat of plants for glass production, cement furnaces,steel furnaces and other processes with waste heat of a temperaturegreater than 150° C. The utilization of geothermal heat, solar heat orthe waste heat from the incineration of waste, biomass and other liquidor solid fuels, or combined operation of gas turbines and waste heatutilization, is likewise possible by means of an ORC process.

Geothermal heat can either be utilized directly or be converted toelectrical energy. The conversion of the geothermal heat to electricalenergy is becoming ever more significant. Typically, steam-poweredprocesses operated with an organic working fluid (ORC working fluid) areused here. Such a system with an organic working fluid is composed ofthe main components of evaporator, turbine, condenser and feed pump, andthe accompanying control and regulation elements. The working fluid isselected such that its thermodynamic properties are adjusted to the heatsource present. Preference is given to using working fluids whichevaporate at comparatively low temperatures.

The geothermal heat is utilized as a heat source in the form of hotthermal water with a temperature of, for example, approx. 180° C.Thermal water with a lower temperature is likewise utilizable as a heatsource, in which case a working fluid with appropriately low evaporationtemperature has to be selected.

The working fluid is preheated by the heat source, then superheated andfinally decompressed in a turbine and, as this is done, via a shaft,drives a generator for electricity generation. Subsequently, the workingfluid is condensed and compressed again by means of the feed pump andrecycled into the preheater.

In order to guarantee economically viable operation, particularrequirements are made on the working fluid. In particular, the workingfluids are selected taking account of the heat content of the heatsource.

The working fluid should be non-toxic and not have any ozone-depletingpotential. Moreover, it should as far as possible not be combustible andalso have a still appropriately low pressure at high temperatures. Theworking fluid should have a high thermal and chemical stability. Sinceworking fluids are used in closed circuits which typically consist ofmetallic components, the question of reactivity and of corrosionbehaviour toward the metals used is likewise an important aspect in theselection of the working fluids.

To limit the capital costs for the turbines, the use of working fluidswith high molecular weight has been found to be advantageous. Owing totheir relatively low speed, the utilizable kinetic energy of heavymolecules can largely be dissipated with one turbine blade ring or onlya few turbine blade rings at maximum turbine speed. In the case oflighter and hence more mobile fluids, in the acquisition, either moreexpensive larger turbines or two-stage turbines are required to achievehigh rotational speeds, or additional gears have to be incorporated.

Examples of suitable working fluids which have already been used includehydrocarbons such as pentane, or halogenated hydrocarbons such astrifluoromethane, tetrafluoromethane, pentafluoropropane (EP 0 066 439,U.S. Pat. No. 6,880,344). A significant disadvantage in the case of useof pure hydrocarbons is their explosiveness.

It is an object of the invention to provide a working fluid for energyrecovery, especially for utilization of heat sources having atemperature up to approx. 200° C., preferably up to approx. 180° C.,which satisfies the requirements mentioned.

It is another object of the invention to provide a method for energyrecovery, especially for utilization of heat sources having atemperature up to approx. 200° C., preferably up to approx. 180° C.,which satisfies the requirements mentioned.

Still another subject of the present invention is to provide anapparatus for performing an ORC process.

These and other objects of the present invention are achieved by virtueof the working fluid comprising at least one compound selected from thegroup of the partially or perfluorinated hydrocarbons such astetrafluoroethane, especially 1,1,1,2-tetrafluoroethane,pentafluoropropane, especially 1,1,1,3,3-pentafluoropropane,hexafluoropropane, heptafluoropropane, especially1,1,1,2,3,3,3-heptafluoropropane, pentafluorobutane, especially1,1,1,3,3-pentafluorobutane, hexafluorobutane, heptafluorobutane,decafluoropentane, perfluoropentane, perfluorohexane or mixturesthereof, and/or at least one compound from the group of the partially orperfluorinated polyethers and/or of the partially or perfluorinatedketones; by a ORC process for energy recovery, preferably forutilization of heat sources having a temperature up to approx. 200° C.,preferably up to approx. 180° C., in which a working fluid comprising atleast one compound selected from the group of the partially orperfluorinated hydrocarbons such as tetrafluoroethane, especially1,1,1,2-tetrafluoroethane, pentafluoropropane, especially1,1,1,3,3-pentafluoropropane, hexafluoropropane, heptafluoropropane,especially 1,1,1,2,3,3,3-heptafluoropropane, pentafluorobutane,especially 1,1,1,3,3-pentafluorobutane, hexafluorobutane,heptafluorobutane, decafluoropentane, perfluoropentane, perfluorohexaneor mixtures thereof, and/or at least one compound from the group of thepartially or perfluorinated polyethers and/or of the partially orperfluorinated ketones is applied; and by the apparatus comprising theORC working fluid according to the present invention.

In the following, the ORC working fluids are described in detail. Thepreferred embodiments of the ORC working fluids as described in detailbelow are also preferred embodiments of the ORC process in which theyare applied, and of the apparatus in which they are comprised.

Mixtures of the fluorinated hydrocarbons mentioned above, with thefluorinated polyethers and/or ketones mentioned above may likewise beused as the working fluid for an ORC process and are preferred asworking fluids according to the present invention. The inventive workingfluid is selected from the multitude of suitable compounds or mixturesof these compounds such that the boiling point of the working fluid usedis below the temperature of the heat source, so that the evaporation ofthe working fluid is possible without any great technical complexity.The composition of the mixtures is preferably selected such that anazeotropic or virtually azeotropic mixture is formed.

The working fluid used is stable within the temperature range ofinterest and has good thermodynamic properties.

Suitable perfluorinated polyethers are described, for example, in WO02/38718. These perfluorinated polyethers consist essentially of carbon,fluorine and oxygen atoms and comprise at least two, preferably three,C—O—C ether linkages, or a mixture of several compounds satisfying thatdefinition. Often, the oxygen atoms in the perfluoropolyether areexclusively present within the C—O—C ether linkages. Theperfluoropolyethers generally have a molecular weight of about 200 ormore. Generally they have a molecular weight of less than about 1500. Ifthe polyether is a mixture of several substances, the molecular weightis the weight-average molecular weight. Generally, theperfluoropolyether has a boiling point greater than or equal to 40° C.at 101.3 kPa. The perfluoropolyether generally has a boiling point lessor equal to about 200° C. at 101.3 kPa. As a result of the preparation,these perfluoropolyethers often are a mixture of individual substances.

Generally, the kinematic viscosity of the perfluoropolyether is lessthan or equal to 1 cSt (Centistoke) at 25° C. Generally, the kinematicviscosity is at least 0.3 cSt at 25° C.

The preferred perfluoro polyethers used are the products marketed bySolvay Solexis under the names GALDEN® and FOMBLIN®.

Examples include:

GALDEN® HT 55: boiling point 57° C. at 101.3 kPA; average molecularweight 340

GALDEN® HT 70: boiling point 66° C. at 101.3 kPa; average molecularweight 410

FOMBLIN® PFS1: boiling point 90° C. at 101.3 kPa; average molecularweight 460

The suitable partially fluorinated polyethers used may be thehydrofluoro ethers marketed by 3M under the name NOVEC®. The GALDEN® andFOMBLIN® systems are usually multicomponent systems having a boilingpoint in the range from 40 to 76° C.

In a preferred embodiment of the invention, the partially fluorinatedhydrocarbons used are especially 1,1,1,3,3-pentafluorobutane(HFC-365mfc), 1,1,1,2-tetrafluoroethane (HFC 134a),1,1,1,3,3-pentafluoropropane (HFC 245fa) or their mixtures with oneanother.

The partially or perfluorinated polyethers used are preferably GALDEN®HT55, GALDEN® HT 70.

Mixtures of the polyethers with the fluorinated hydrocarbons arelikewise suitable working fluids in the context of the invention.

The fluorinated ketones used are partially or perfluorinated ketones ofthe general formula R—C(O)—R′ where R and R′ are partially orperfluorinated substituents which may be the same or different and arepreferably fluorinated alkyl groups. However, R may also be a linear orbranched alkyl group. The fluorinated alkyl groups having preferably 1to 6 carbon atoms may likewise be linear or branched, in which case notmore than two fluorine atoms may be replaced by hydrogen. R ispreferably perfluoroisopropyl and R′ is preferably a trifluoromethyl orpentafluoroethyl group. In one embodiment, the partially fluorinatedketone used is a compound of the abovementioned general formula in whichR is a linear or branched alkyl group, preferably a methyl group, and R′is as defined above. Mixtures of the ketones with the fluorinatedhydrocarbons are likewise suitable. The preparation of these fluorinatedketones is described in EP 1 261 398. The boiling points of the suitableketones are within the range from 0° C. to about 150° C., preferably inthe range from 0° C. to about 110° C., in particular in the range from0° C. to about 75° C.

In a preferred embodiment, fluorinated ketones from the group ofCF₃C(O)CF(CF₃)₂, CF₃CF₂C(O)CF(CF₃)₂, CH₃C(O)CF₂CF₂H, CH₃C(O)CF₂CFHCF₃are used. In a further preferred embodiment, the ketones are used as aworking fluid in combination with partially or perfluorinatedhydrocarbons. In particular, mixtures which comprise or consist ofHFC-365mfc and at least one compound of the ketones mentioned,preferably CF₃C(O)CF(CF₃)₂, are used.

A preferred embodiment concerns mixtures comprising one or morehydrofluorocarbons and one or more (partially and/or per-)fluorinatedethers and/or one or more (partially and/or per-)fluorinated ketones.Especially preferred are such mixtures comprising1,1,1,3,3-pentafluorobutane (HFC-365mfc) and/or1,1,1,3,3-pentafluoropropane (HFC 245fa) as hydrofluorocarbons(s).

Compositions described in WO 02/38718 are especially suitable as workingfluids according to the present invention.

In the compositions described therein, the weight ratio of thehydrofluoroalkane to the perfluoropolyether is generally greater than orequal to 5:95, often greater than or equal to 10:90, preferably greaterthan or equal to 25:75. The weight ratio of the hydrofluoroalkane to theperfluoropolyether is generally less than or equal to 95:5, often lessthan or equal to 90:10, preferably less than or equal to 85:15.

The binary azeotropes or pseudo-azeotropes formed from HFC-365mfc andespecially the perfluoropolyethers GALDEN® HT55, GALDEN® HT70 orFOMBLIN® PFS1, which are described in WO 02/38718, are examples forsuitable mixtures useful as ORC working fluids according to the presentinvention.

In a further especially preferred embodiment, the working fluid used isa mixture of HFC-365mfc and GALDEN® HT 55 in a mixing ratio of 65 to35%.

Mixtures of HFC-365mfc and hydrofluoroethers or HFC-365mfc andperfluorohexane or HFC-365mfc and perfluoropentane or HFC-365mfc anddecafluoropentane are likewise preferred working fluids, especially forenergy conversion using geothermal water as the heat source. The mixingratio may be different and, for example, be 50/50, 40/60 or 65/35.Advantageously, the mixing ratio should be selected such that anazeotropic or virtually azeotropic mixture is formed.

The inventive working fluids may especially be used for energyconversion starting from heat sources having a temperature of about 50to 180° C. They are particularly suitable for conversion of geothermalheat, in the form of thermal water, to electrical energy. Energyconversion utilizing other heat sources, such as solar heat or wasteheat from refuse incineration, biomass or other liquid or solid fuels islikewise conceivable. Suitable biomass includes, for example, biomassfrom plants or feces from animal husbandry.

Likewise, it is possible to convert waste heat, e.g. from industrialprocesses, according to the present invention. In another embodiment ofthe present invention, the conversion is performed in an installation orplant for combined heat and power, e.g. in a communal heating/powerstation.

The working fluids mentioned above are suitable for coolingphoto-voltaic cells for increasing the conversion of solar energy. Theheat energy absorbed thereby can be utilized for example, according tothe present invention, for conversion into additional electric energy.Cooling can be effected here separately, e.g. via a flange-mountedstructural element, but as well by direct immersion of the photo-voltaiccells into the working fluid (immersion cooling).

Further, the working fluids according to the present invention can beapplied to use heat sources directly in work machines or processingmachines, such as, for example, decentralized work or processingmachines, especially autarchic pumps in which, for example, solar energyis converted into mechanical energy. The term “decentralized” means inthis context especially that the work or processing machine is suppliedessentially according to the present invention with energy from a heatsource which naturally occurs at the location of the work or processingmachine, or which was created artificially for the needs of the work orprocessing machine.

Combination of a plurality of process stages, for example in the form ofsecondary cycles, allows the efficiency of the plants to be increased.The working fluids used in the individual secondary cycles may be thesame or different. In this case, the waste heat of the individualsecondary cycles of the cycle process is sent in each case to the nextprocess or the next stage. In the case of use of different workingfluids, the working fluids may be selected such that they have differentvapour pressures as a function of the temperature, and the vapourpressures should each be within the optimal range. As a result of such aseries connection of secondary circuits, the possibility exists ofderiving heat energy, for example for heating purposes, after eachstage.

In one embodiment, the thermal water is delivered into the plant with adepth pump. Here, the heat is released to the working fluid in apreheater and an evaporator. The working fluid circulates in a so-calledsecondary circuit, where, after the pressure increase by the feed pump,it passes through the preheater and the evaporator. The superheatedsteam is decompressed in the turbine and drives, via a shaft, agenerator for electricity generation. The decompression does not proceedabove the condensation curve, but rather always remains outside the wetsteam range. The decompressed fluid is thus still superheated and thisheat must be removed before the working fluid condenses and it is passedback to the preheater. The removed heat can for example be used to heatrooms.

The preferred working fluids, especially a mixture of 60 to 70% byweight, preferably 65% by weight, of HFC-365mfc, and 30 to 40% byweight, preferably 35% by weight, GALDEN® HT55 perfluoropolyethers, areespecially suitable for all of the applications described above.

An apparatus, comprising the ORC working fluid according to the presentinvention, suitable for performing the ORC process of the presentinvention is also subject of the present invention. Such apparatus inprinciple are known. They usually comprise means to heat up the ORCworking fluid, e.g. an evaporator, optionally a preheater and/orsuperheater, a turbine which is connected to a generator for producingelectrical current, and a heat consumer (condenser) for the ORC workingfluid.

FIG. 1 gives a sketch of an apparatus according to the invention. Itcomprises an evaporator E, a line 1 which is connected to the evaporatorand a heat consumer HC tvia a pump P in which the ORC fluid iscompressed. Through line 1, liquid ORC working fluid is transported fromthe heat consumer HC to the evaporator E. The evaporator E is connectedthrough a line 2 with a turbine T. Through line 2, ORC working fluid istransported in vapour form to the turbine T. Turbine T is connected to agenerator G which produces electrical energy. The turbine T is connectedto the heat consumer HC via line 3. The vapour leaving the turbinethrough line 3 is condensed in the heat consumer HC. Evaporator E can beheated by respective heat sources like geothermal water, biomass etc.The apparatus according to the invention comprises one of theabove-mentioned ORC working fluids. The heat generated in the heatconsumer HC can for example be used for heating rooms.

The invention will be illustrated below using a working example.

EXAMPLE 1

The ORC process starts from an external heat supply via a preheater andan evaporator.

The heat source used was geothermal water having a temperature of 100°C. The fluid, a mixture of HFC-365mfc and GALDEN® HT 55 (mixing ratio65/35% by weight) evaporates in the evaporator. The fluid vapour passesto a turbine which drives a generator for electricity generation. On theoutput side, the fluid was conducted into a condenser (heat consumer)and the fluid condensed here was recycled back into the preheater.

1. A working fluid for an ORC process, comprising or consisting of atleast one compound selected from the group of the partially orperfluorinated hydrocarbons and/or at least one compound from the groupof the partially or perfluorinated polyethers and/or at least onecompound from the group of the partially or perfluorinated ketones or amixture of these compounds with one another.
 2. A working fluid for anORC process according to claim 1, wherein the partially orperfluorinated hydrocarbon is at least one compound from the group oftetrafluoroethane, pentafluoropropane, hexafluoropropane,heptafluoropropane, pentafluorobutane, hexafluorobutane,heptafluorobutane, decafluoropentane, perfluoropentane, perfluorohexaneor mixtures thereof.
 3. A working fluid according to claim 2 furthercomprising at least one partially or perfluorinated polyethers.
 4. Aworking fluid according to claim 1, wherein the partially orperfluorinated polyethers have a boiling point greater than or equal to40° C. at 101.3 kPa.
 5. A working fluid according to claim 1, whereinthe partially or perfluorinated ethers have a boiling point equal to orless than about 200° C. at 101.3 kPa.
 6. A working fluid according toclaim 1, comprising or consisting of a mixture of1,1,1,3,3-pentafluorobutane and at least one perfluorinated polyether.7. A working fluid according to claim 1, wherein the at least oneperfluorinated polyether has a boiling point of about 57° C. at 101.3kPa.
 8. A working fluid according to claim 1, wherein the partially orperfluorinated ketones are selected from compounds having the generalformula R—C(O) —R′, where R is an alkyl group, or a linear or branchedfluorinated C1-C6-alkyl group in which not more than two fluorine atomsmay be replaced by hydrogen, and R′ is a linear or branched fluorinatedC1-C6 alkyl group in which not more than two fluorine atoms may bereplaced by hydrogen.
 9. A working fluid according to claim 1, whereinthe fluorinated ketones are selected from the group consisting ofCF₃C(O)CF(CF₃)₂, CF₃CF₂C(O)CF(CF₃)₂, CH₃C(O)CF₂CF₂H, CH₃C(O)CF₂CFHCF₃.10. A working fluid according to claim 9, comprising or consisting of amixture of 1,1,1,3,3-pentafluorobutane and CF₃C(O)CF(CF₃)₂.
 11. An ORCprocess for converting heat into electrical energy wherein an ORCworking fluid according to claim 10 is used as ORC working fluid.
 12. Anapparatus for performing an ORC process comprising an ORC working fluidaccording to claim
 1. 13. The working fluid for an ORC process accordingto claim 2, wherein the partially or perfluorinated hydrocarbon is atleast one compound from the group of 1,1,1,2-tetrafluoroethane,1,1,1,3,3-pentafluoropropane, hexafluoropropane,1,1,1,2,3,3,3-heptafluoropropane, 1,1,1,3,3-pentafluorobutane,hexafluorobutane, hepafluorobutane, decafluoropentane, perfluorohexaneor mixtures thereof.
 14. The working fluid according to claim 8, whereinthe partially or perfluorinated ketones are selected from compoundshaving the general formula R—C(O)—R′, where R is a methyl group, and R′is a linear or branched fluorinated C1-C6 alkyl group in which not morethan two fluorine atoms may be replaced by hydrogen.